Projection exposure apparatus with at least one manipulator

ABSTRACT

A projection exposure apparatus for microlithography includes a projection lens which includes a plurality of optical elements for imaging mask structures onto a substrate during an exposure process. The projection exposure apparatus also includes at least one manipulator configured to change, as part of a manipulator actuation, the optical effects of at least one of the optical elements within the projection lens by changing a state variable of the optical element along a predetermined travel. The projection exposure apparatus further includes an algorithm generator configured to generate a travel generating optimization algorithm, adapted to at least one predetermined imaging parameter, on the basis of the at least one predetermined imaging parameter.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit under 35 U.S.C. §119 of German patentapplication DE 10 2012 205 096.5, filed Mar. 29, 2012, the entirecontents of which are hereby incorporated by reference.

FIELD

The disclosure relates to a projection exposure apparatus formicrolithography and a method for operating such a projection exposureapparatus. A projection exposure apparatus for microlithography is usedto create structures on a substrate in the form of a semiconductor waferduring the production of semiconductor elements. The projection exposureapparatus includes a projection lens with a plurality of opticalelements for imaging mask structures on the wafer during an exposureprocess.

BACKGROUND

To ensure very precise imaging of the mask structures on the wafer, aprojection lens with the fewest possible wavefront aberrations isdesired. Projection lenses are therefore equipped with manipulatorswhich render it possible to correct wavefront aberrations by changingthe state of individual optical elements of the projection lens.Examples of such a state change include a change in position in one ormore of the six rigid body degrees of freedom of the relevant opticalelement, an application of heat and/or coldness to the optical element,and a deformation of the optical element. Usually, the aberrationcharacteristic of the projection lens is measured at regular intervalsand, if desired, changes in the aberration characteristic are determinedbetween the individual measurements by simulation. Thus, for example,lens-element heating effects can be taken into account in calculations.The terms “lens heating”, “lens-element warming”, “mirror heating” and“mirror warming” are also used synonymously for “lens-element heating”.The manipulator changes to be carried out to correct the aberrationcharacteristic are calculated using a travel generating optimizationalgorithm, which is also referred to as “manipulator changing model”.

“Travel” is understood to mean a change in a state variable of anoptical element, effected via manipulator actuation, for the purpose ofchanging the optical effects thereof. Such travel defined by changing astate variable of the optical element is specified by intended changevariables of the manipulator. By way of example, the manipulation canconsist of a displacement of the optical element in a specificdirection, but also, for example, of a local or two-dimensional loadingof the optical element with heat, coldness, forces, light of a specificwavelength or currents. By way of example, the intended change variablecan, in the case of a displacement, define a path length to be travelledor an angular range to be travelled.

The desire for continuous miniaturization of the structures to be imagedand for increasing the throughput lead to the situation where theaberration characteristic cannot, in general, be corrected sufficientlysatisfactorily in a conventional manner by manipulators, neither duringoperation nor over the service life of a projection exposure apparatus.In this case, “sufficiently satisfactorily” is understood to mean thatthe uncorrected residual aberration characteristic leads to a sufficientimaging quality for a multiplicity of usage configurations. Here, ausage configuration should be understood to mean a combination of a maskwith an illumination setting used for imaging the same.

It was found to be desirable to set the residual aberrationcharacteristic in view of the processed usage configurations so thatimportant structures, in particular so-called core-region structures(described in more detail below), are imaged very precisely and so thatthe residual structures, i.e. the peripheral structures, are imaged lessprecisely but still sufficiently precisely. This is possible, amongother reasons, because core-region structures and peripheral structuresscan different regions of the wavefronts and the peripheral structurespose significantly lower demands on the scanned wavefront region.

It is known to use travel generating optimization algorithms whichusually solve unrestricted quadratic optimization problems inregularized fashion, usually via matrix multiplication by a singlepreceding calculation of the inverse. To regularize so-called “ill-posedproblems”, use is made in particular of singular value decompositionwith singular-value cut-off or Tikhonov regularization. Details aredisclosed in, for example, “Iterative Methods for III-Posed Problems: AnIntroduction,” Inverse and III-Posed Problems, B. Bakushinsky, Mihail Y.Kokurin and Alexandra Smirnova, De Gruyter, 2010, chapters 4 and 5,pages 23-43. Here, the respective user configuration can, as a matter ofprinciple, not be taken or not be sufficiently taken into account.

SUMMARY

The disclosure provides a projection exposure apparatus and a method foroperating such a projection exposure apparatus with which imagingaberrations occurring during an exposure process are able to becorrected with great accuracy within short time intervals. The imagingaberrations occurring during an exposure process are desirably correctedwithin short time intervals with the accuracy that is involved for thesuccessful imaging process with respect to the different demands forcore-region structures and peripheral structures.

In one aspect, the disclosure provides a projection exposure apparatusfor microlithography including a projection lens which includes aplurality of optical elements for imaging mask structures onto asubstrate during an exposure process. The apparatus also includes atleast one manipulator configured to change, as part of a manipulatoractuation, the optical effects of at least one of the optical elementswithin the projection lens by changing a state variable of the opticalelement along a predetermined travel. The apparatus further includes analgorithm generator configured to generate a travel generatingoptimization algorithm, adapted to at least one predetermined imagingparameter, on the basis of the at least one predetermined imagingparameter. The at least one imaging parameter includes structureinformation with respect to mask structures to be imaged during asubsequent exposure process and/or structure information with respect toan angular distribution of exposure radiation radiated onto the maskstructures during the subsequent exposure process. The apparatus alsoincludes a travel establishing device configured to establish at leastone travel for a manipulator actuation via the travel generatingoptimization algorithm. As mentioned above, the travel defines a changein a state variable of the optical element. This change is effected bythe manipulator actuation. The travel establishing device is thereforeconfigured to establish at least one travel for a change in the statevariable of the optical element effected by manipulator actuation viathe travel generating optimization algorithm.

In other words, a travel generating optimization algorithm is first ofall generated in a targeted fashion when the projection exposureapparatus is operated. The algorithm is adapted to at least one imagingparameter which is predetermined for the subsequent exposure process.This specifically adapted optimization algorithm, also referred to as“manipulator change model”, is then used to calculate travel correctionsfor at least one manipulator of the projection lens.

Such a manipulator is configured, as part of a manipulator actuation, tochange the optical effects of at least one of the optical elementswithin the projection lens by changing a state variable of the opticalelement along a predetermined travel. By way of example, such amanipulator actuation can include a positional change of the opticalelement in one of the six rigid body degrees of freedom, an applicationof heat and/or coldness to the optical element, and/or a deformation ofthe optical element. The predetermined travel along which such amanipulator actuation is brought about is defined by the manipulatoractuation of successively passed through states in the manipulated statevariable of the optical element. In the case of a positional change bytranslation in space, the travel is a path in three-dimensional space.In the case of an application of heat and/or coldness to the opticalelement, a travel can, for example, be defined by a temporal successionof successively assumed temperature states.

The generation of the travel generating optimization algorithm iseffected on the basis of at least one predetermined imaging parameter,more particularly an imaging parameter set, which includes informationwith respect to the mask structures and/or information with respect toan illumination stetting for an upcoming exposure process. In generalterms, the manipulator changing model is created on the basis ofstructure information with respect to mask structures to be imagedand/or structure information with respect to an angular distribution ofthe exposure radiation radiated onto the mask. The angular distributionof the exposure radiation radiated onto the mask is also referred to as“illumination calibration” or “illumination setting” below. By way ofexample, the structure information with respect to the mask structuresto be imaged can include a line width, a characterization of thegeometric structure of the mask structures, an orientation of structuresin the core region and/or the periphery of a semiconductor chip.Examples of frequently used illumination settings include annularillumination, dipole illumination and quadrupole illumination.

As a result of the generation of the travel generating optimizationalgorithm on the basis of at least one predetermined imaging parameter,the optimization algorithm can be adapted so well to the characteristicof the subsequent exposure process that the optimization algorithm makesdo with comparatively few computational operations. As a result, itbecomes possible to provide travel signals for the at least onemanipulator during the exposure process at short time intervals, as aresult of which the aberration characteristic of the projection lens canbe corrected with great accuracy. Here, it is not necessarily allaberration parameters that are corrected uniformly, but rather those arecorrected in a targeted fashion which are relevant to the at least oneimaging parameter.

In accordance with one embodiment, the projection exposure apparatusincludes a storage device for storing a set of aberration parameterswhich in general terms define the imaging quality of the projectionlens. In accordance with a further embodiment, the optimizationalgorithm is configured to set the at least one travel so that, in thecase of a corresponding actuation of the manipulator, a subset of theaberration parameters is optimized. Here the subset, also referred to as“selected subset” below, is in particular reduced compared to thecomplete set of aberration parameters by at least one aberrationparameter, the influence of which on the imaging behaviour of theprojection exposure apparatus in the predetermined imaging parameter isless than the respective influence of the remaining aberrationparameters. In doing so, this does not preclude the residual subset fromlikewise experiencing an optimization. However, in this case theselected subset is particularly optimized, i.e., optimized to a greaterextent, than the residual subset. By way of example, this can beimplemented by virtue of the regions of the wavefronts scanned by thecore-region structures being intended to satisfy tighter specificproperties with respect to the imaging behaviour than the regions of thewavefronts scanned by the peripheral structures. By way of example,aberration parameters can be Zernike coefficients, weighted sums ofZernike coefficients, lithographic variables such as, e.g., alithographically measured astigmatism or a lithographically measuredcoma aberration, imaging variables such as overlay, variation of thebest focus and/or fading effects. The definition of such effects isprovided in, for example, pages 30-33 of WO 2010/034674 A1.

As opposed to the peripheral structures, the core-region structures aredefined on the mask. Here, the core-region structures are thosestructures that are imaged with a sufficiently small pitch or with thesmallest pitch. The sufficiently small pitch is defined by virtue of thefact that those structures whose pitch is less than or equal to thesufficiently small pitch make up at least 5%, in particular at least10%, at least 50% or at least 80% of all structures situated on the maskto be imaged. The peripheral structures are then all structures whichare not core-region structures. It was found that, in general, thecore-region structures react sensitively to small changes in specificregions of the wavefront, whereas these are insensitive to relativelylarge changes in other regions. Here, a high sensitivity leads to highyield losses in the production of the corresponding semiconductor chips.

By reducing the set of optimized aberration parameters or the set ofespecially optimized aberration parameters to the aforementioned subset,it becomes possible to establish a manipulator correction, adapted in atargeted fashion to the imaging process, in a particularly short timeand simple fashion. The set of aberration parameters by which theimaging quality of the projection lens is determined merely includesaberration parameters whose influence on the imaging quality issignificant for the purpose of the projection lens. In accordance withone embodiment, those aberration parameters have a significant influenceon the imaging quality which, overall, make up at least 50%, inparticular at least 90%, at least 95% or at least 99% of a lithographicerror observed overall in core-region structures. Here, as alreadymentioned previously, lithographic error observed overall is understoodto mean, for example, a coma aberration, an overlay error, variations ofthe best focus and/or fading effects in the core-region structures. Inparticular, the aforementioned set includes those aberration parameterswhich are optimized by conventional optimization methods.

In accordance with a further embodiment, the travel generatingoptimization algorithm is configured to establish the travel for themanipulator actuation on the basis of at least one aberration parameterwhich characterizes the imaging quality of the projection lens. Here,the travel is established in such a way that the imaging quality isimproved in the case of a change of the corresponding state variable ofone of the optical elements along the travel.

In accordance with a further embodiment, the travel generatingoptimization algorithm is based on a mathematical model with at most1000, in particular at most 500, at most 250, at most 100, at most 60,at most 40 or at most 25 basis functions. Such a model renders itpossible to generate current travel commands within a short period oftime. In accordance with one embodiment, the travel establishing deviceis configured to establish the at least one travel in less than 500 ms,in particular less than 100 ms or less than 20 ms. In accordance withone embodiment configured for an EUV projection exposure apparatus, thetravel is established in less than 30 seconds, in particular less than10 seconds. In accordance with a further embodiment, the travelgenerating algorithm is configured to carry out matrix multiplicationsand, in particular, uses as a basis a singular value decomposition or aTikhonov regularization, in particular with forming an inverse.

In accordance with a further embodiment, the algorithm generator has adatabase with a plurality of different stored algorithms. In accordancewith one variant, the algorithm generator is configured to select one ofthe stored algorithms as travel generating optimization algorithm on thebasis of the predetermined imaging parameter.

In accordance with a further variant, the algorithm generator isconfigured to adapt a stored algorithm stored in the algorithm generatorto the predetermined at least one imaging parameter. The storedalgorithm can be an algorithm which is already adapted to a certaindegree to the predetermined imaging parameter. This can be an algorithmwhich was optimized in preliminary fashion at an earlier time or analgorithm which is partly or wholly adapted to a similar imagingparameter. Alternatively, it is also possible to use a standardalgorithm which is not specifically adapted to an imaging parameter. Inaccordance with a further embodiment, the algorithm generator isconfigured to effect the adaptation of the stored algorithm to thepredetermined imaging parameter by carrying out an optimization method.To this end, the algorithm generator in particular includes a so-calledalgorithm optimizer.

In accordance with a further embodiment, the optimization method servingto adapt the stored algorithm is based upon a merit function, whichtakes into account the influence of a change in the imaging quality ofthe projection lens due to lens-element heating, also referred to as“lens heating”, directly or indirectly during the exposure process on atleast one lithographic error. That is, the influence of the abovespecified change on at least one lithographic mirror is taken intoaccount. Here, a lithographic error should be understood to mean anerror which occurs during lithographic imaging, such as, for example, aso-called overlay error. An overlay error specifies a localimage-position displacement of an imaged mask structure compared to theintended position thereof on the substrate. The imaging quality can beinfluenced indirectly by way of, for example, suitable weightings on oddZernike coefficients or weighted linear combinations of odd Zernikecoefficients if use is made of a manipulator changing model based on themethod of least squares. Thus, for example, such a manipulator changingmodel can be based on singular value decomposition or Tikhonovregularization.

In accordance with a further embodiment, the travel establishing deviceis configured to activate, in an exposure pause, a travel generatingoptimization algorithm, which is newly generated by the algorithmgenerator, and therefore use it for establishing the travel from theactivation time onward. An exposure pause can be a pause during which abatch interchange is carried out. A batch is understood to mean wafersexposed as packet, which are generally exposed with uniform illuminationparameters, i.e., with the same mask and the same illumination setting.An exposure pause serving to activate the optimization algorithm canalso be the short period of time between the end of the exposure of apreceding wafer and the start of the exposure of the next wafer. Thisperiod of time can also be used for measuring purposes.

In accordance with a further embodiment, the projection exposureapparatus includes a sensor for measuring an external physical variable,and the travel establishing device is configured to take into accountthe external physical variable when establishing the travel. Inparticular, there is a continuous adaptation of the travel to thephysical variable. An example of such a physical variable is thesurrounding air pressure.

In accordance with a further embodiment, the projection exposureapparatus includes a simulation device, which is configured to simulatechanges in the optical properties of the optical elements, which changesoccur as the result of heating of the optical elements, for example onthe basis of the radiation effect during the exposure process or due toadditional lens-element heating for achieving an optimum operatingtemperature with respect to the material quality in the case of EUVprojection exposure apparatuses. Here, the optimization algorithm isconfigured to calculate the at least one travel on the basis of thesimulated heating-induced changes in the optical properties.

In accordance with a further embodiment, the projection exposureapparatus is configured to successively image the mask structuresrespectively onto different regions of the substrate in a plurality ofexposure steps and the travel establishing device is configured toestablish an updated version of the at least one travel after everyexposure step. In other words, the travel is updated after every exposedfield.

In one aspect, the disclosure provides a method for operating aprojection exposure apparatus for microlithography. The apparatusincludes a projection lens having a plurality of optical elements forimaging mask structures onto a substrate during an exposure process. Themethod includes predetermining at least one imaging parameter for asubsequent exposure process. The imaging parameter includes structureinformation with respect to the mask structures to be imaged and/orstructure information with respect to an angular distribution ofexposure radiation radiated onto the mask structures. The method alsoincludes generating a travel generating optimization algorithm adaptedto the predetermined imaging parameter on the basis of the at least onepredetermined imaging parameter. The method further includesestablishing at least one travel for at least one of the opticalelements via the travel generating optimization algorithm. The traveldefines a change of a state variable of the at least one opticalelement. In addition, the method includes actuating the at least oneoptical element along the predetermined travel by changing the statevariable.

In accordance with one embodiment, the travel generating optimizationalgorithm, adapted to the predetermined imaging parameter, is generatedby the projection exposure apparatus. In accordance with an alternativeembodiment, the travel generating optimization algorithm, adapted to thepredetermined imaging parameter, is generated outside of the projectionexposure apparatus and, after it has been generated, is read into theprojection exposure apparatus. It is optionally possible to provide theuser with the option of approving the optimization algorithm oractivating it after reading in the adapted optimization algorithm. Inaccordance with one variant, the projection exposure apparatus can havean algorithm generator to which an imaging parameter and/or a travelgenerating algorithm are/is transmitted by the user.

In accordance with a further embodiment, at least one control parameterdefining the adapted optimization algorithm is generated outside of theprojection exposure apparatus and, after it has been generated, is readinto the projection exposure apparatus. The optimization algorithm hasone or more merit functions, as well as constraints. Control parametersof the aforementioned type denote those parameters which, in the case ofthe same initial state, change one or more of the merit functions or oneor more of the constraints.

The features specified with respect to the embodiments, listed above, ofa projection exposure apparatus can be correspondingly applied to amethod. Conversely, the features specified with respect to theembodiments, listed above, of a method according can be correspondinglyapplied to a projection exposure apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantageous features of the disclosure areillustrated in the following detailed description of exemplaryembodiments with reference to the accompanying schematic drawings, inwhich:

FIG. 1 depicts an embodiment of a projection exposure apparatus formicrolithography with an algorithm generator for generating a travelgenerating optimization algorithm; and

FIG. 2 is a flowchart which depicts an embodiment of the functionalityof the algorithm generator.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the exemplary embodiments described below, elements which arefunctionally or structurally similar to one another are as far aspossible provided with the same or similar reference signs. Therefore,for understanding the features of the individual elements of a specificexemplary embodiment, reference should be made to the description ofother exemplary embodiments or the general description of thedisclosure.

To facilitate the description of the projection exposure apparatus, aCartesian xyz-coordinate system is indicated in the drawing, whichsystem reveals the respective positional relationship of the componentsillustrated in the figures. In FIG. 1, the y-direction runsperpendicularly to the plane of the drawing out of the latter, thex-direction runs towards the right and the z-direction runs upwards.

FIG. 1 shows an embodiment of a projection exposure apparatus 10 formicrolithography. The embodiment is designed for operation in the EUVwavelength range, i.e. with electromagnetic radiation with a wavelengthof less than 100 nm, in particular a wavelength of approximately 13.5 nmor approximately 6.7 nm. As a result of this operating wavelength, alloptical elements are embodied as mirrors. However, the disclosure is notrestricted to projection exposure apparatuses in the EUV wavelengthrange. Further embodiments according to the disclosure are designed, forexample, for operating wavelengths in the UV range, such as e.g. 365 nm,248 nm or 193 nm. For such embodiments, at least some of the opticalelements are configured as conventional transmission lenses.

The projection exposure apparatus 10 in FIG. 1 includes an exposureradiation source 12 for generating exposure radiation 14. In the presentcase, the exposure radiation source 12 is embodied as an EUV source andcan, for example, include a plasma radiation source. The exposureradiation 14 first of all passes through an illumination optical unit 16and is directed onto a mask 18 from the latter. The illumination opticalunit 16 is configured to generate different angular distributions of theexposure radiation 14 incident on the mask 18. Depending on anillumination adjustment, also referred to as “illumination setting”,desired by the user, the illumination optical unit 16 configures theangular distribution of the exposure radiation 14 incident on the mask18. Examples of selectable illumination settings include a so-calleddipole illumination, annular illumination and quadrupole illumination.

The mask 18 has mask structures for imaging a substrate 24 and isdisplaceably mounted on a mask displacement stage 20. The mask 18 can beembodied as a reflection mask, as illustrated in FIG. 1, oralternatively, in particular for UV lithography, also be configured as atransmission mask. In the embodiment in FIG. 1, the exposure radiation14 is reflected at the mask 18 and thereupon passes through a projectionlens 22, which is configured to image the mask structures on thesubstrate 24. The substrate 24 is displaceably mounted on a substratedisplacement stage 26. The projection exposure apparatus 10 can beembodied as a so-called scanner or as a so-called stepper. The exposureradiation 14 is routed within the illumination optical unit 16 andwithin the projection lens 22 via a multiplicity of optical elements,which are shown as mirrors in FIG. 1.

In FIG. 1, the projection lens merely has four optical elements E1 toE4. All optical elements are mounted in a movable fashion. Moreover, arespective manipulator M1 to M4 is associated with each of the opticalelements E1 to E4. Each manipulator M1, M2 and M3 enables a displacementof its respective optical elements E1, E2 and E3 in the x- andy-direction and hence a displacement substantially parallel to the planein which the respective reflective surface of the optical element lies.

The manipulator M4 is configured to tilt the optical element E4 byrotation about a tilt axis 27 which is arranged parallel to the y-axis.As a result, the angle of the reflecting surface of E4 is changed withrespect to the incident radiation. Further degrees of freedom for themanipulators are feasible. Thus, for example, provision can be made fora transverse displacement of the relevant optical element with respectof the optical surface thereof, or a rotation about a reference axisperpendicular to the reflecting surface.

In general terms, each of the manipulators M1 to M4 is provided foreffecting a displacement of the associated optical element E1 to E4 byperforming a rigid body movement along a predetermined travel. By way ofexample, such a travel can combine translations in different directions,tilts and/or rotations in arbitrary fashion, or else can consist of adifferent natured change in a state variable of the associated opticalelement by corresponding actuation of the manipulator.

The projection exposure apparatus 10 furthermore includes a centralcontrol device 28. The central control device 28 controls the variouscomponents of the projection exposure apparatus 10 when carrying out anexposure process. In order to prepare an exposure process, the centralcontrol device 28 transmits mask-selection information 32 to the maskdisplacement stage 20. The mask displacement stage 20 thereupon takesthe mask desired for the subsequent exposure process from a mask libraryand loads the mask into the exposure position.

Furthermore, the central control device 28 transmits illuminationsetting information 30 to the illumination optical unit 16. Theillumination setting information 30 defines the angular distributionwith respect to the exposure radiation 14 radiated onto the mask,desired for the subsequent illumination process. As already mentionedabove, this angular distribution is often also referred to as“illumination setting”. The illumination optical unit 16 undertakescorresponding settings of the desired illumination setting.

The projection exposure apparatus 10 furthermore includes a manipulatorcontrol 40 for controlling the manipulators M1 to M4. The manipulatorcontrol 40 in turn includes an algorithm generator 42 and a travelestablishing device 44. The algorithm generator 42 is configured togenerate a travel generating optimization algorithm 52 and transmit thelatter to the travel establishing device 44. The travel establishingdevice 44 uses the travel generating optimization algorithm 52 forestablishing control signals which serve to control the manipulators M1to M4, as will be described in more detail below. The central controldevice 28 provides the algorithm generator 42 with an imaging parameterset with respect to an upcoming exposure process.

In accordance with one embodiment variant, the algorithm generator 42can also be arranged outside of the projection exposure apparatus 10.The manipulator control 40 in this case includes a read-in device forreading in the travel generating optimization algorithm 52.Alternatively, it is also possible that only control parameters of theoptimization algorithm 52 are generated outside of the projectionexposure apparatus 10 and transmitted to the manipulator control 40.

The imaging parameter set includes at least one item of structureinformation with respect to the mask structures to be imaged and/orstructure information with respect to the illumination setting. Inaccordance with one embodiment, the imaging parameter set includesillumination setting information 30 and mask structure information 46.Within the scope of this application, such an imaging parameter set isalso referred to as “usage configuration”. The illumination settinginformation 30 identifies the angular distribution of the exposureradiation 14 radiated onto the mask 18. The mask structure information46 can include a precise geometric reproduction of the mask structuresto be imaged, or else only specify essential structure aspects of same,such as e.g. line width, characterization of the geometric structures ofthe mask structures, orientation of mask structures, for exampledifferentiated with respect to central structures and peripheralstructures.

In the following text and with reference to FIG. 2, an embodimentaccording to the disclosure of the procedure of the algorithm generator42 for generating the travel generating optimization algorithm 52 willbe described on the basis of the imaging parameters, provided by thecentral control device 28, in the form of the illumination settinginformation 30 and the mask structure information 46.

First of all, the algorithm generator 42 accesses a database 48, inwhich a multiplicity of stored algorithms 50, which are adapted todifferent imaging parameter sets, are stored. The algorithm generator 42initially checks whether the database 48 already contains a storedalgorithm 50 which is adapted to the predetermined imaging parameters.

If a suitably adapted stored algorithm 50 is available in the database48, the algorithm generator 42 transmits the former to the travelestablishing device 44 as travel generating optimization algorithm 52.If this is not the case, the algorithm generator 42 selects one of thestored algorithms 50 from the database 48 as a start algorithm for asubsequent optimization method. The start algorithm can either be ageneral standard algorithm, or else an algorithm which is optimized withrespect to an imaging parameter set which comes close to thepredetermined imaging set. Furthermore, the database 48 can also have astored algorithm 50 which is already optimized in a preliminary fashionto the predetermined imaging parameter set. In this case, the algorithmgenerator 42 can select this algorithm as start algorithm.

In order to adapt the selected start algorithm to the predeterminedimaging parameters, a load parameter collection is initially establishedin a step S1. This load parameter collection represents a collection ofaberration parameter sets, which are expected to occur during theoperation of the projection lens 22. The aberration parameter sets arealso referred to as “load cases”. When generating the aberrationparameter sets, changes in the aberration parameters as a result ofthermal effects in the form of lens-element heating during the exposureprocess are initially taken into account.

The aberration parameter sets describe the imaging quality of theprojection lens 22 and, in accordance with one embodiment, include a setof Zernike coefficients. A set of measured aberration parameters in theform of Zernike coefficients, stored in a storage device 56, is firstlyretrieved in step S1. Thereupon changes in the imaging parameter set,which are expected to occur during the upcoming exposure process as aresult of lens-element heating and optionally other deterministicoperational-dependent influences, are established by simulation. Thesechanges are determined dependent on the imaging parameters predeterminedfor the upcoming exposure step. In other words, the characteristic ofthe lens-element heating and optional other deterministicoperational-dependent influences under the predetermined imagingparameters are calculated in a targeted fashion during the simulationand the changes in the imaging parameter set resulting therefrom areestablished. All imaging parameter sets occurring during the duration ofthe upcoming exposure process form the aforementioned load parametercollection.

Thereupon, the load parameter collection is complemented in step S2 byfurther aberration parameter sets that are expected to occur. Examplesof such complements include modifications of the aberration parametersin the case of changing air pressure, influences of elementary imageaberrations, such as e.g. scale errors and/or aberration parameterchanges, which occur during the imaging of general relevant structures.These complementary aberration parameter sets in particular include loadcases which are independent of the illumination setting and can beinterpreted as “basic requirements” for the correction capability of theprojection lens.

In the now following step S3, respective conversion parameter sets areestablished for converting the aberration parameter sets of the loadparameter collection into lithographic errors. “Lithographic errors” areunderstood to be errors of the projection lens, which can be measured inthe lithographic image, i.e. in the aerial image present in thesubstrate plane or in the structure generated on the substrate 24 by thelithographic imaging on the photoresist. Such lithographic errors arealso referred to as imaging parameter errors and are in contrast towavefront aberrations, which cannot be measured directly in thelithographic image. An example of such a lithographic error is aso-called “overlay error”. As already mentioned above, the overlay errorrepresents a local image position displacement of an imaged maskstructure compared to the intended position thereof on the substrate.

The conversion parameter sets established as per step S3 render itpossible to convert the aberration parameter sets into selectedlithographic errors, in particular overlay errors.

The established conversion parameter sets are generally available in theform of linear factor datasets. Such linear factor datasets can berepresented by matrices which, field point by field point, convert theZernike coefficients characterizing the wavefront into lithographicerror variables.

Thereupon a merit function for an optimization method for generating anoptimized travel generating optimization algorithm 52 is generated in astep S4. The merit function is designed in such a way that selectedlithographic errors, in particular overlay errors, receive suitableweighting for the subsequent optimization.

Thereupon, the travel generating optimization algorithm 52 is determinedin a step S5 by optimization on the basis of the merit functiongenerated in step S4. As already explained above, the travel generatingoptimization algorithm is, as per one variant, created on the basis ofsingular value decomposition or Tikhonov regularization. Here, theTikhonov weightings or the singular value parameters, for example, arereleased for optimization, with the stipulation that the properties ofthe load cases are satisfied to the best possible extent. The storedalgorithm 50 taken from the database, which can either be a standardalgorithm or an algorithm already optimized in a preliminary fashion,serves as a start algorithm, as mentioned previously. For theoptimization method used in the process, use can be made of variousbasic algorithms known to a person skilled in the art, for examplesimulated cooling, also known as “simulated annealing”, geneticalgorithms and/or evolutionary algorithms and convex programming, inparticular sequential quadratic programming (SQP). With respect to thelatter methods, reference is made to Stephen Boyd, Lieven Vandenbergh,“Convex Optimization”, Cambridge University Press (2004), chapter 4.4,pages 152-153, chapter 4.6, page 167, chapter 4.7, pages 174-184,chapters 11.1-11.5, pages 561-596, and chapter 11.8, pages 615-620, andalso to Walter Alt, “Nichtlineare Optimierung” [Nonlinear optimization],Vieweg, 2002, chapter 8, pages 291-305.

The travel generating optimization algorithm 52 generated by thealgorithm generator 42 is transmitted to the travel establishing device44 and stored in the database 48 as further stored algorithm. Theoptimization algorithm 52 is used by the travel establishing device 44to determine travels 45 for the individual manipulators M1 to M4. Thetravels 45 define changes to be carried out in the corresponding statevariables of the optical elements E1 to E4. The established travels 45are transmitted to the individual manipulators M1 to M4 via travelsignals and predetermine correction travels respectively to be carriedout by these. These define corresponding displacements of the associatedoptical elements E1 to E4 for correcting wavefront aberrations of theprojection lens 22 which have occurred. In order to establish thetravels 45, the travel establishing device 44 obtains respectivelyupdated aberration parameters 64 of the projection lens 22 whilecarrying out the exposure process. These aberration parameters 64 can,for example, include the wavefront characterizing Zernike coefficients.

The travel generating optimization algorithm 52 generated by thealgorithm generator 42 is adapted to the imaging parameter set utilizedin the exposure process, i.e. in particular to the illumination settingand the utilized mask structures, in such a way that the travel can beestablished without delays that interfere with the exposure process.Thus, the travel establishing device 44 as per one embodiment which isdesigned for operating projection exposure apparatuses in the UVwavelength range generates updated travels 45 a number of times persecond, for example every 100 milliseconds, and therefore updatestravels in real time. Updating the travels a number of times per secondfor example renders it possible to readjust the manipulators after everyfield exposure. In an embodiment adapted for EUV projection exposureapparatuses, it is optionally possible to allow longer periods of timeto pass between the updates, for example 30 seconds.

The optimization algorithm 52 is based on a mathematical model with atmost 1000 basis functions. In accordance with various embodiments, theoptimization algorithm is based on at most 400, at most 100 or at most60 basis functions. In accordance with one embodiment, the travels 44are established by matrix multiplications with an inverse matrixcalculated in advance, as is possible, for example, in the case of anapproach on the basis of the singular value decomposition or Tikhonovregularization. In this case, m×n is supposed to be the matrix size.From this, m•n matrix multiplications follow, where m is typicallyformed from the product of the specified number of Zernike coefficientsand the number of measured field points. Examples for the specifiednumber of Zernike coefficients include 36, 49, 64 or 100 Zernikecoefficients, while a value of between 30 and 100 is suitable for thenumber of measured field points. If the Zernike coefficient Z1 isignored, the specified values with respect to the Zernike coefficientsreduce by one. The parameter “n” represents the number of the overallpossible degrees of freedom of the manipulator. By way of example, if 49Zernike coefficients are measured or simulated or extrapolated atrespectively 65 field points, and 52 degrees of freedom are allowed forthe manipulators, this results in 165 620 matrix multiplications forgenerating the travels 45.

The optimization algorithm 52 can be based on algorithms which are wellknown to a person skilled in the art, e.g. on singular valuedecomposition, also referred to as “SVD”, and/or Tikhonovregularization. In both cases, the problem is reduced to carrying out amatrix multiplication of the form x=Ap by calculating an inverse matrixor a pseudo-inverse matrix. Here, x is the change of the manipulatormanipulated vector, p is the interference to be corrected and A is asuitable matrix, in general the suitably regularized inverse of asensitivity matrix.

The regularly updated aberration parameters of the projection lens 22are transmitted to the travel establishing device 44 by an aberrationparameter transmitter 54. The aberration parameter transmitter 54 has astorage device 56 and a simulation device 58. Aberration parameters 64are stored in the storage device 56, which parameters were establishedusing a wavefront measurement at the projection lens 22. Thesemeasurement results can be collected using an external wavefrontmeasuring instrument. However, alternatively, the aberration parameters54 can also be measured by a wavefront measurement device 55 integratedin the substrate displacement stage 26. By way of example, such ameasurement can be taken regularly after each exposure of a wafer orrespectively after exposing a complete wafer set. Alternatively, asimulation or a combination of simulation and reduced measurement can beundertaken instead of a measurement.

The measured values of the aberration parameters 64 stored in thestorage device 56 are optionally adapted to respective updatedconditions during the exposure process by the simulation device 58. Inaccordance with one embodiment, the current radiation intensity 62 is tothis end regularly transmitted to the simulation device 58 by thecentral control device 28. From this, the simulation device 58calculates changes in the aberration parameters 64 effected due tolens-element heating on the basis of the respective illuminationsetting. Furthermore, the simulation device continuously obtainsmeasurement values from a pressure sensor 60 which monitors the pressuresurrounding the projection exposure apparatus 10. Effects of changes inthe surrounding pressure on the aberration parameters 64 are taken intoaccount by the simulation apparatus 58.

The generation and use of a travel generating optimization algorithm 52is automatically monitored by a suitable entity. In the case ofirregularities, a switch is made to emergency operation on the basis ofa standard optimization algorithm which is suitable for a multiplicityof imaging parameter sets. Irregularities can be detected on the basisof the analysis of travel commands or the analysis of residualwavefronts. Thus, for example, a malfunction and hence the presence ofirregularities can be deduced from observing oscillating travelcommands.

The optimization of a stored algorithm in the algorithm generator 42with respect to an imaging parameter set, described on the basis of FIG.2, can also occur during an exposure break or else during the imagingoperation of the projection exposure apparatus 10. Thus, for example,the exposure operation can initially be started with a travel generatingstandard algorithm, and a stored algorithm can be adapted to theutilized imaging parameter set during the operation. When the adaptedoptimization algorithm is completed, the latter is then taken up by thetravel establishing device 44 during an exposure pause.

In some embodiments, various implementations of the systems and methodsdescribed here can be realized in computing devices such as digitalelectronic circuitry, integrated circuitry, specially designed ASICs(application specific integrated circuits), computer hardware, firmware,software, and/or combinations thereof. These various implementations caninclude implementation in one or more computer programs that areexecutable and/or interpretable on a programmable system including atleast one programmable processor, which may be special or generalpurpose, coupled to receive data and instructions from, and to transmitdata and instructions to, a storage system, at least one input device,and at least one output device.

These computer programs (also known as programs, software, softwareapplications or code) include machine instructions for a programmableprocessor, and can be implemented in a high-level procedural and/orobject-oriented programming language, and/or in assembly/machinelanguage. As used herein, the terms “machine-readable medium” and“computer-readable medium” refer to any computer program product,apparatus and/or device (e.g., magnetic discs, optical disks, memory,Programmable Logic Devices (PLDs)) used to provide machine instructionsand/or data to a programmable processor, including a machine-readablemedium that receives machine instructions as a machine-readable signal.The term “machine-readable signal” refers to any signal used to providemachine instructions and/or data to a programmable processor.

LIST OF REFERENCE NUMERALS

-   10 Projection exposure apparatus-   12 Exposure radiation source-   14 Exposure radiation-   16 Illumination optical unit-   18 Mask-   20 Mask displacement stage-   22 Projection lens-   24 Substrate-   26 Substrate displacement stage-   E1, E2, E3, E4 Optical elements-   M1, M2, M3, M4 Manipulators-   27 Tilt axis-   28 Central control device-   30 Illumination setting information-   32 Mask selection information-   40 Manipulator control-   42 Algorithm generator-   44 Travel establishing device-   45 Travel-   46 Mask structure information-   48 Database-   50 Stored algorithm-   52 Travel generating optimization algorithm-   54 Aberration parameter transmitter-   55 Wavefront measuring device-   56 Storage device-   58 Simulation device-   60 Pressure sensor-   62 Radiation intensity-   64 Aberration parameter

What is claimed is:
 1. An apparatus, comprising: a projection lenscomprising a plurality of optical elements configured to image maskstructures onto a substrate during an exposure process, the plurality ofoptical elements comprising a first optical element; a manipulatorconfigured to change, as part of a manipulator actuation, opticaleffects of the first optical element by changing a state variable of thefirst optical element along a predetermined travel; a first computingdevice configured to generate a travel algorithm based on at least oneimaging parameter, the at least one imaging parameter comprisingstructure information with respect to mask structures to be imagedduring a subsequent exposure process and/or structure information withrespect to an angular distribution of exposure radiation radiated ontothe mask structures during the subsequent exposure process; and a secondcomputing device configured to establish a travel for a manipulatoractuation based on the travel algorithm, wherein the apparatus is amicrolithography projection exposure apparatus.
 2. The apparatus ofclaim 1, further comprising a storage device configured to store a setof aberration parameters which in general terms define an imagingquality of the projection lens, wherein the travel algorithm isconfigured to set the travel so that, in the case of a correspondingactuation of the manipulator, a subset of the aberration parameters isoptimized.
 3. The apparatus of claim 1, wherein the travel algorithm isconfigured to establish the travel for the manipulator actuation basedon at least one aberration parameter which characterizes an imagingquality of the projection lens.
 4. The apparatus of claim 1, wherein thetravel algorithm is based on a mathematical model with at most 1000basis functions.
 5. The apparatus of claim 1, wherein the firstcomputing device comprises a database with a plurality of differentstored algorithms.
 6. The apparatus of claim 5, wherein the firstcomputing device is configured to select one of the plurality ofdifferent stored algorithms as the travel algorithm based on the atleast one predetermined imaging parameter.
 7. The apparatus of claim 1,wherein the first computing device is configured to adapt a storedalgorithm stored in the first computing device to the at least onepredetermined imaging parameter.
 8. The apparatus of claim 7, whereinthe first computing device is configured to effect the adaptation byperforming an optimization method.
 9. The apparatus of claim 8, whereinthe optimization method is based on a merit function which takes intoaccount an influence of a change in imaging quality of the projectionlens due to lens heating during the exposure process.
 10. The apparatusof claim 1, wherein the second computing device is configured toactivate, in an exposure pause, a travel algorithm which is newlygenerated by the first computing device.
 11. The apparatus of claim 1,further comprising a sensor configured to measure an external physicalvariable, wherein the second computing device is configured to take intoaccount the external physical variable when establishing the travel. 12.The apparatus of claim 1, further comprising a simulation deviceconfigured to simulate changes in the optical properties of the opticalelements, wherein the changes occur due to heating of the opticalelements, and the travel algorithm is configured to calculate the travelbased on the simulated heating-induced changes in the opticalproperties.
 13. The apparatus of claim 1, wherein the second computingdevice is configured to establish the travel in less than 500 ms. 14.The apparatus of claim 1, wherein the apparatus is configured tosuccessively image the mask structures respectively onto differentregions of the substrate in a plurality of exposure steps and the travelestablishing device is configured to establish an updated version of theat least one travel after every exposure step.
 15. The apparatus ofclaim 1, wherein the apparatus comprises a device which comprises thefirst and second computing devices.
 16. The apparatus of claim 1,further comprising an illumination optical unit configured to illuminatethe mask structures during use of the apparatus.
 17. A method foroperating a microlithography projection exposure apparatus, theapparatus comprising a projection lens having a plurality of opticalelements configured to image mask structures onto a substrate during anexposure process, the method comprising: determining an imagingparameter for a subsequent exposure process, the imaging parametercomprising structure information with respect to the mask structures tobe imaged and/or structure information with respect to an angulardistribution of exposure radiation radiated onto the mask structures;generating a travel algorithm adapted based on the at least onepredetermined imaging parameter; using the travel algorithm to establisha travel for at least one of the optical elements, the travel defining achange of a state variable of the at least one optical element; andactuating the at least one optical element along the by changing thestate variable.
 18. The method of claim 17, comprising using theprojection exposure apparatus to generate the travel algorithm.
 19. Themethod of claim 17, comprising: generating the travel algorithm outsideof the projection exposure apparatus; and reading the travel algorithminto the projection exposure apparatus.
 20. The method of claim 17,comprising: generating a control parameter defining the travel algorithmoutside of the projection exposure apparatus; and reading the controlparameter into the projection exposure apparatus.
 21. The method ofclaim 17, wherein the projection exposure apparatus further comprises: amanipulator configured to change, as part of a manipulator actuation,optical effects of the first optical element by changing a statevariable of the first optical element along a predetermined travel; afirst computing device configured to generate a travel algorithm basedon at least one imaging parameter, the at least one imaging parametercomprising structure information with respect to mask structures to beimaged during a subsequent exposure process and/or structure informationwith respect to an angular distribution of exposure radiation radiatedonto the mask structures during the subsequent exposure process; and asecond computing device configured to establish a travel for amanipulator actuation based on the travel algorithm.